Lipids (oils or fats), important nutrients in addition to proteins and carbohydrates, are particularly useful as an energy source. It however has a high calorie (9 kcal/g) and intake of it promotes obesity and can be causative of problems such as life-style related diseases. A meal rich in lipids are typically delicious and people of the present day are accustomed to such a meal. In advanced countries under satiation, an increase in lipidic intake has come to be a serious nation-wide problem together with a rise in medical expenses. In recent days, people are highly interested particularly in health promotion and maintenance and preventive treatment of diseases and a number of investigations have been made on the relationship between lipids and obesity or life-style related diseases.
Primary investigations to date have been related to fatty acids constituting a triglyceride, a principal component of a lipid. Nutritionally essential fatty acids are, for example, linoleic acid, arachidonic acid and linolenic acid. These fatty acids are known to be utilized in the body as a constituent of a biomembrane or a raw material of eicosanoids (prostaglandin, thromboxanes, leukotrienes, etc.). In addition, it is reported that there is a high possibility of saturated fatty acids in a diet having a blood-serum cholesterol heightening action, leading to atherosclerosis or heart diseases (Lancet, 2, 959(1950)); and a high linoleic acid oil contained much in a diet increases tumor incidence and size of experimental animals (J. National Cancer Institute, 66, 517(1971)). It is described that an oleic-acid-rich and saturated-fatty-acid-poor diet lowers LDL-cholesterol level, while maintaining an HDL-cholesterol level, thereby reducing the risk of heart diseases (J. Lipid Res., 26, 194(1985), New England J. Medicine, 314, 745(1988)). In addition, physiological activities of various ω3-unsaturated fatty acids including antithrombus effects of eicosapentaenoic acid contained in a fish oil have drawn attention (Ann. Rev. Nutr., 8, 517(1988)). On the other hand, owing to many double bonds, eicosapentaenoic acid or docosahexaenoic acid involves a problem in stability against oxidation; and eicosapentaenoic acid has an anticoagulant action. Based on the study of intake balance of these fatty acids, a number of research reports have been presented, for example, on a recommendable ratio of saturated fatty acid:mono-unsaturated fatty acid:polyunsaturated fatty acid or a ratio of 6-unsaturated fatty acid: ω3-unsaturated fatty acid. Research is still in progress (Nutrition and Diseases of Oils and Fats”, published by Saiwai Shobo, The 6th edition of Recommended Dietary Allowances for Japanese”, Ministry of Health and Welfare).
With a view to preventing obesity, substitutes for fats and oils or non-absorptive fats and oils have been developed and typical ones include sucrose fatty acid polyester (U.S. Pat. No. 3,600,186). It is excreted without being absorbed in the body so that the calorie derived from fat is 0 kcal/g. There is however a potential problem that it causes anal leakage and inhibits absorption of fat-soluble vitamins. In addition, it is not a supply source of essential fatty acids. The use of this substance was authorized by FDA in 1996, under the restriction that a semi-solid or solid sucrose fatty acid polyester having a melting point of 37.8 to 71.1° C. and containing predetermined amounts of vitamins A, D, E and K can be used only for salty snack foods. This restriction is made for preventing both anal leakage and inhibition of absorption of fat-soluble vitamins. It is reported that a medium-chain fatty acid triglyceride (MCT) is not accumulated on the body, but it has poor heat stability. Similar effects of conjugated linoleic acid, fish oil or perilla oil are disclosed (Lipids, 32, 853(1997), J. Agric. Food Chem., 46, 1225(1998)).
Further disclosed with attention paid to the glyceride structure is an edible oil composition (EP Patent No. 0525,915), a cholesterol-level lowering agent (Japanese Patent No. 2035495), an agent for lowering the triglyceride concentration in serum (Japanese Patent Application Laid-Open No. 4-300825), a body weight increase inhibitor (Japanese Patent Application Laid-Open No. 4-300826), a preventive or therapeutic agent for fatty liver (Japanese Patent Application Laid-Open No. 4-300828) and a liquid general-purpose oil/fat composition (U.S. Pat. No. 6,004,611). They paid attention only to the effect derived from the diglyceride structure. Although they are free of problems such as anal leakage or inhibition of fat-soluble vitamin absorption, they do not induce effects of diglyceride at the maximum. In addition, when they are used as a processing edible oil, when heated at a high temperature for long hours, thus being exposed to severe conditions, their acid value (AV) tends to show a slight increase.
An object of the present invention is to provide an extremely useful oil/fat composition which has good heat stability, which can be used widely for various purposes, which can prevent obesity due to oils or fats and contributes to good health.